The invention relates to a color display tube comprising
an envelope with a longitudinal axis, having a neck portion, a funnel portion and a window portion; PA1 an electron gun arranged in the neck portion, PA1 a display screen having a short central axis and a long central axis and a pattern of phosphor elements (for example in the form of rows) on the inner surface of the window portion; PA1 a color selection means arranged proximate to the display screen; PA1 a funnel-shaped shield of a magnetically permeable material having two long wall portions parallel to the long axis of the display screen and two short wall portions parallel to the short axis of the display screen, and an aperture at the gun-sided end, which aperture extends transversely to the longitudinal axis and constitutes an aperture for passing electron beams produced by the gun and scanning the display screen. PA1 1. welding on the strips is a relatively expensive operation, PA1 2. the spot welds are not very reliable (loosening), PA1 3. oil and grease residues behind the welded strips are difficult to remove (risk of cathode poisoning). PA1 local deformation (by means of a centre punch, a powder spray method or laser beam radiation) in the area between the aperture and the edge; PA1 local diffusion of a non-magnetic material suitable for use in an evacuated space, such as Al ,CrNi, Mn, C or N, in the area between the aperture and the edge (note that Al requires a higher temperature during the diffusion process than the other materials mentioned); PA1 local non-decarbonizing annealing by coating a part of the wall surface during annealing; PA1 suppressing the crystal growth by local full annealing before the magnetic annealing step; PA1 local cooling during the magnetic annealing step.
A color selection means is herein understood to mean, for example an apertured shadow mask sheet or a wire mask.
The ratio between the dimension of the long central axis and the dimension of the short central axis of the display screen characterizes the picture format.
In a (color) display tube the earth's magnetic field deflects the electron paths, which without any measures may be so large that the electrons impinge upon the wrong phosphor (mislanding) and produce a discoloration of the picture.
Modern display tubes are provided with an internal magnetic shield to limit the deviation of the electron path due to the earth's magnetic field. A complete shielding is not possible due to an aperture which is required for the passage of the electron beam. A horizontally directed spot displacement caused by the lateral earth's magnetic field produces a risk of discoloration in the corners only (N effect). It is known from U.S. Pat. No. 4,758,193 that the internal residual field can be influenced by means of an additional measure in such a way that the electron beam still passes the mask at the desired angle. This measure involves the use of a shield with "vertically" directed slits (situated in a plane through the longitudinal axis parallel to the short axis of the display screen). The internal residual field is then influenced in such a way that there is less spot displacement in the horizontal direction. The slits enhance the magnetic resistance in the shield in the horizontal direction so that there is more spot displacement in the vertical direction. However, for display tubes having phosphor rows extending in this direction this is not important because it does not lead to discoloration.
A problem of "vertically" directed slits is that the slit length is to be limited to ensure the mechanical stability of the shield so that an unacceptable spot displacement remains in the corners, particularly in large tubes. In extremely large tubes having, for example a picture diagonal of 41 cm or more, such as 80 cm FS ("Flat Square") and 36 inch WS ("Wide Screen"), i.e. the ratio between the short central axis of the display screen and the long central axis of the display screen is 9:16, it has been attempted to lengthen the slits to a maximum extent and to restore the resultant loss of mechanical strength by welding on strips of a non-ferromagnetic material which bridge the slits. However, the following problems then occur.
It is an object of the invention to provide a display tube of the type described in the opening paragraph in which the earth's magnetic field is shielded as satisfactorily as in the known display tube without detrimentally influencing the mechanical stability of the shield, even in larger tubes having a picture diagonal from, for example 41 cm onwards.